Fuel cell power plants require a gas flow circuit to provide oxygen bearing gas (usually air) for process feed to the cathode side of the fuel cell where a portion of the oxygen is consumed and to provide cooling of the fuel cell.
The pressure in this flow circuit is maintained by a compressor that draws air from the ambient atmosphere and compresses it to that of the flow circuit which is typically several atmospheres. Under steady state operating conditions oxygen depleted air is exhausted from the circuit subsequent the fuel cell at the compressor charging rate.
Although the circuit pressure is several atmospheres and the pressure drop around the circuit is only about one atmosphere the mass flow rate around the circuit is usually more than an order of magnitude greater than the mass flow rate of the compressor charging the circuit. The circuit temperature meanwhile is on the order of several hundred degrees Fahrenheit.
While fans for circulating air across a pressure differential of one psi with a flow rate in excess of one million pounds (mass) per hour are readily and commercially available for use at atmospheric pressures, fans with these performance characteristics that operate in a high pressure, high temperature environment of several atmospheres and several hundred degrees Fahrenheit and operated by an electric motor or steam turbine require a special housing or casing, are complex, and difficult to design. A high pressure housing or casing for the fan is necessary along with either an external steam or electric motor operating through a sealed rotating shaft or an internal electric motor that is specially cooled. Gas or liquid and electrical penetrations into the operating system would prove complex and expensive as well as providing potential failure modes.
The flow circuit for a gas cooled phosphoric acid fuel cell (PAFC) power plant consists of circulating air for both cooling and process feed to the cathode side of the fuel cell where a portion of the air is consumed. Make up air is supplied to this circuit by a compressor which compresses ambient air from 14.6 psia to 50 psia at a flow rate of 64,320 pounds mass per hour. The air flow rate within the circulating loop is much higher at 1,039,000 pounds mass per hour. A fan or circulator must drive this circulating air across a pressure differential of approximately 1.0 psia. This 1.0 psia pressure differential is due to the fuel cells and the heat exchanger where thermal energy is removed from the circulating air, and the associated air piping and fittings.
The mean pressure and temperature within the air circuit is 50 psia and 271.degree. F. These are nominal values which may vary slightly with actual power plant designs. Fans for circulating air across a pressure differential of 1.0 psia with a flow rate of 1,039,000 pounds mass per hour are not unusual when only a low pressure housing or casing is required for the fan. However the PAFC circulating fan requires a housing or casing suitable for an internal operating pressure of 50 psia which is unusual given present design and construction practices for commercially available fans and blowers. The PAFC power plant, therefore, will require a fan in a configuration that is not available commercially at present.
If a special high pressure casing for the fan is used and it is driven from an electric motor or steam turbine through a sealed rotating shaft, the shaft seal and bearings constitute potential sources of failure in the operating PAFC power plant. The shaft bearings and seal could be eliminated by constructing the fan and an electric motor entirely within the circulating air duct, but a special electric motor would be required to operate in an ambient air condition of 271.degree. F. and either liquid or gas cooling may be required for the motor. Electric wiring leads for the motor will also add complexity, cost, and present potential failure modes. In addition a special enclosed motor designed for high temperature operation possibly with its own cooling system would be expensive.
U.S. Pat. No. 3,473,963 discloses a fuel cell that circulates oxygen bearing process gas by means of a constant volume air pump driven by an electric motor external to the flow system.
U.S. Pat. No. 3,576,677 discloses a fuel cell that circulates oxygen bearing process gas by means of a fan driven by an electric motor. The entire motor-fan system is inside the flow circuit requiring a casing to withstand the circuit pressure and the motor must be capable of operating at system temperature.